This invention relates to a process for the production of fiber-reinforced molded products based on polyurethanes.
In the present state of the art, fiber-reinforced molded products may be produced from many different synthetic polymers. Glass fibers have proved to be a particularly suitable reinforcing material. It is known, for example, to produce semifinished goods (resin mats "pre-pregs", molding materials) which are stable in storage from resins containing unsaturated polyesters (hereinafter referred to as "UP resins"). These substances may be shaped in heated presses and cured to form molded bodies with good strength and rigidity. However, this method has various disadvantages. The vinyl group-containing monomers (e.g. styrene) normally used as solvents for the UP resins cause a very high density of cross-linking in radical-initiated polymerization which, in turn, causes brittleness and sensitivity to notched impact in the molded product. The presence of solvents is an inconvenience in processing since it requires elaborate measures for removal of the solvent vapors by suction and the provision of processing plants which are protected against explosion.
To produce the semifinished product, the viscosity of the UP resin must be increased so that the reinforcing fibers will be transported without difficulty in the subsequent heating and pressing process. Small quantities of an alkaline earth metal oxide or hydroxide are usually added to the resin for this purpose. The addition of such a compound starts a lengthy ripening process which normally takes from 7 to 21 days and must be carried out at a slightly elevated temperature in so-called "ripening cupboards." This interim storage entails considerable expenditure in time and capital outlay and renders the process relatively uneconomical.
It is also known to use fiber materials to reinforce polyurethane elastomers. Glass fiber-reinforced polyurethane molded products, for example, may be produced by the injection molding or reaction injection molding process. The mechanical properties of these molded products, however, particularly the flexural strength thereof, may be only increased to a limited extent by this method since, for process technical reasons, the lengths of the fibers must not exceed from ca. 1 to 6 mm. It must be regarded as particularly disadvantageous that, due to the limited lengths of the fibers, the thermal coefficient of expansion of the reinforced polyurethane elastomers is still a multiple of the coefficient of expansion of steel.
A process for the production of high molecular weight cross-linked synthetic resins is described in German Pat. No. 968,566. In this process, an intermediate product is first produced from a hydroxyl polyester, a glycol and a subequivalent quantity of diisocyanate and this intermediate product is then reacted with an excess of a diisocyanate containing uretdione groups to produce a semifinished product which is stable in storage. The semifinished products obtained by this method may subsequently be subjected to a plastic shaping process and cured by the action of heat to produce elastic molded articles.
The polyurethane elastomers described in the aforesaid reference have a predominantly linear structure and, although they have a high elasticity and toughness, the hardness and rigidity thereof are insufficient for many purposes. The use of reinforcing material in the form of fibers is not mentioned in German Pat. No. 968,566. Although glass fibers longer than 6 mm could, in principle, be incorporated in the above-mentioned intermediate product, the high viscosity of the intermediate product would make it necessary to use rollers or a kneader of the type normally used for rubber processing in order to incorporate the fibrous material and the uretdione diisocyanate necessary for cross-linking. If such a method were used for incorporating the fibrous material, the shearing forces produced would be so high that the individual fibers would be broken down to fractions of the original length so that the desired effects of an increase in rigidity and a reduction in the coefficient of thermal expansion would no longer be obtained to the desired extent.
It is an object of the present invention to provide a process for producing fiber-reinforced molded products with good rigidity, impact strength and dimensional stability within economically acceptable periods of time, i.e., without prolonged interim storage of the intermediate product.
This problem is solved by the process according to the present invention.